Update Claude Code plugin for current bifrost tooling

[foundev]

The plugin launched the MCP server via the removed uvx brokk bifrost entry point and its agents/skills described the bifrost v0.1.3 tool set. Update everything to bifrost 0.6.3 as launched by uvx brokk mcp:

• .mcp.json: launch via uvx brokk mcp (default searchtools profile exposes the full tool set, so no flags needed)
• Agents: replace the get_symbol_locations+Grep caller-tracing idiom with scan_usages, route text/file/git lookups through search_file_contents, find_filenames, get_file_contents and the git MCP tools, and add targeted code-quality tools (structural clones for dry-reviewer, secret scanning for security, god-object/complexity reports and symbol ancestors for architect, jq/xml tools for devops)
• Skills: rewrite code-navigation, code-reading, codebase-search and git-exploration around the new tools; drop the now-false "no caller-graph / no git tools / no file-contents tool" caveats; remove the no-longer-existing get_symbol_summaries; fix the most_relevant_files parameter name; make workspace activation conditional since a workspace is active at server startup; point "embedded prompt" fallbacks at the agents/ directory
• brokk-core/README.md: drop the broken uvx brokk mcp-core quick start and note the plugin now uses bifrost